1. Field of the Invention
The present invention relates to an optical signal quality monitor device, and particular to a monitor device applicable, for example, to an optical transmission system.
2. Description of the Background Art
In an optical transmission system, the quality of a transmitted optical signal is monitored to reflect results from the monitoring on an optical signal receiver or the like.
The signal quality is monitored, for example, by a parity check between repeaters, thus locating the failure section or producing a signal for switching to an alternative section. The parity check detects errors after demodulated from the optical signal. Therefore, when the transmission section varies its quality very rapidly, the parity check result may not correctly reflect the quality of the subject section.
The quality of the optical signal may therefore preferably be monitored substantially on a real time basis. Japanese patent laid-open publication No. 2005-151597 proposes an optical signal quality monitor that satisfies such a request. The '597 Japanese publication discloses an optical signal quality monitor including a photoelectric converter, a timing clock generator, and an electrical signal processor. The photo electric converter converts an optical signal having its bit rate equal to a positive integer (N) multiple of the basic clock frequency to a corresponding electrical signal. The timing clock generator generates a timing clock signal having its repetition frequency slightly different from a positive integer (n) submultiple of the basic clock frequency. The electrical signal processor samples the level of the electrical signal at the timing clock signal for a predetermined period of time to produce a histogram, and analyze the distribution on the histogram to determine the signal to noise ratio (S/N ratio) to check the quality of the optical signal. The '597 publication thus discloses a technology that performs asynchronous sampling to obtain a waveform in a short period of time, thus monitoring by a relatively simple configuration the Q value, which is a parameter equivalent to the signal to noise ratio.
In the technology of the '597 publication, however, the higher bit rate of a transmitted optical signal, the fewer sampling points, thus making it more difficult to perform the quality monitoring.